1. Field of the Invention
The invention concerns compositions comprising ethylenically unsaturated resin compositions having improved stability with respect to room temperature gel (“RTG”) time drift and stability against gellation compared to conventional systems.
2. The Related Art
Resin compositions, such as unsaturated polyester resin (“UPR”), and vinyl ester resin (“VER”) are typically combined or blended with filler, such as aluminum trihydroxide (“ATH”) with different particle sizes and surface treatment, calcium carbonate, and talc to lower the total cost and achieve desired functions, such as modulus, better resistance to abrasion, and flame retardant properties in case of ATH. Quite often, resin compositions also include rheology modification additives, such as fumed silica with different surface area and surface treatment to achieve desired properties, such as sag resistance. These compositions are used to make molded composite articles. Typically, the molded composite article is formed by contact molding, such as hand layup, spray up, and infusion, which comprises the resin and filler material and a fibrous material, e.g. glass fibers, embedded into a polymer matrix. While the mechanical properties of a bundle of fibers are low, the strength of the individual fibers is reinforced by the polymer matrix that acts as an adhesive and binds the fibers together. The bound fibers provide rigidity and impart structural strength to the molded composite article, while the polymeric matrix prevents the fibers from separating when the molded composite article is subjected to environmental stress.
Obtaining thorough cure and consistent curing behavior in a highly filled system or compositions with rheology modification additives can be a challenge for several reasons. First, the filler acts as a heat sink. This reduces the temperature of the curing composite which both slows the rate of cure and can allow vitrification which nearly stops reaction completely. Cure of composites is most commonly initiated by the catalytic decomposition of hydroperoxides, where metal salts provide the catalysis. These metal salts are present at low levels, and so may be adsorbed onto the surface of inorganic filler or additives. Also, acid or base on the surface of the filler or additive particles or present as an extractable can change the rate of radical production. Such systems are very often subject to changes in gel time on storage.
Several approaches to address the changes in RTG time drift have been proposed in the art and each has several drawbacks. For example, cobalt catalyst and co-promoters may be added to the resin immediately before use. While this approach diminishes, and in some cases eliminates, the problems associated with RTG time drift, the use of cobalt and co-promoters increases complexity of product use at the fabricator. For example, fabricators weigh and manipulate chemicals, and cobalt presents some environmental and toxic challenges. Another approach is to add a partially soluble cobalt compound, commercially known as cobalt 21%. This material slowly releases cobalt on storage, replenishing the cobalt that is absorbed by filler. While sometimes this is difficult to reproduce, in theory this allows the gel time drift to be zero at any point in time—but not at all points. The rates of neither the ‘release’ of new cobalt, nor the ‘deactivation’ of existing cobalt are steady. The two can not be matched. In addition, since this approach utilizes cobalt, careful formulation is needed to minimize RTG time drift and the cobalt presents environmental and toxic challenges. Vanadium has also been used in the art, but this material presents the same drawbacks as cobalt.
All parts and percentages set forth herein are on a weight-by-weight basis except where otherwise specified, for example by weight of the resin, meaning by the amount of all resin (UPR, VER, and/or other resins having ethylenic unsaturation) in the composition.